I'd like to disagree; yes, profit is a large part, but it's a lot less of a hassle than PB. In particular looking for attendants. And as someone who hired Mercs themself in a past battle, man, it's so buttery smooth and easy...

And yeah, Merc companies haven't been there from the beginning, but before that, you still had AH mercs, which is usually more expensive than straight ordering from a Merc Company.